572 R. C. ALLEN 



where exposed in section i8 indicates discordance in trend with the 

 Gwinn series, but too Httle is known of the structure in that vicinity 

 to place any importance on this observation. 



KEWEENAWAN SERIES (?) 



Basic dikes have been cut in a few drill holes and may be 

 observed in section 20, T. 45, R. 25, cutting the basal arkose member 

 of the Gwinn series. These dikes intrude both the Princeton and the 

 Gwinn series. They are younger than Palaeozoic and older than 

 the Princeton series. Their age is therefore probably Keweenawan. 



PALEOZOIC 



Isolated remnants of limestone and sandstone of Cambrian or 

 Ordovician age, or possibly both, occur throughout the district. 

 Some of these are in excess of 50 ft. thick. No fossils or other 

 means of determining the exact age of these outliers is available 

 at the present time. 



CORRELATION OF THE GWINN AND THE PRINCETON SERIES 



It has been shown that the pre-Cambrian sedimentary rocks of 

 the Gwinn synclinorium consist of two unconformable series. The 

 unconformity between them is marked by a basal conglomerate the 

 position, extent, and thickness of which imply an important erosion 

 interval which intervened between the periods of deposition of the 

 two series. 



Concerning the respective ages of these two series, it may be 

 said that probably no geologist familiar with the pre-Cambrian 

 formations of the Lake Superior region would correlate the Gwinn 

 (lower) series with the Lower Huronian. It contains an important 

 iron formation associated with graphitic slates, an assemblage of 

 rocks not known in the Lower Huronian. Moreover, the basal 

 conglomerate carries fragments of quartzite and quartz slate dis- 

 similar to any known Archean sediments in Michigan but exactly 

 similar to certain Lower Huronian rocks in the adjacent Marquette 

 district. This evidence considered in connection with the uncon- 

 formity separating the Princeton and the Gwinn series is a suffi- 

 cient basis for the correlation suggested in this paper, but an 

 additional consideration tending to show that the Gwinn series is 



